Microelectronic devices comprising integrated circuits may comprise stacks of layers. There are many situations in which it is desirable to measure the adhesion strength between two layers of material. Typically, this is a requirement during manufacturing processes of said microelectronic devices, when testing products for reliability. Often the two layers will consist of different materials, but it can also arise that there is an interface between two layers made of substantially the same material.
During manufacture of such microelectronic devices comprising integrated circuits, it is desirable to measure the adhesion strength between layers in a wafer, notably layers deposited as thin films. Ideally this would be done at various locations across the surface of the wafer. Wafer-testing techniques used to date involving stress-measurement techniques such as the curvature method, or x-ray measurement are described in the publication entitled “Intrinsic stress in chemical vapour deposited diamond films: An analytical model for the plastic deformation of the Si substrate” by J. H. Jeong et al, in the Journal of Applied Physics, 90(3), pp 1227-1236, 1 Aug. 2001. However, these techniques only provide information about residual stress in the wafer in general, they do not provide a measurement of adhesion strength between the different layers in the wafer at specific locations on the wafer.
In other fields, various measurement techniques are known which do allow adhesion-strength to be determined. Tests including stress measurement using a microcantilever are known from the publication entitled “Quantitative surface stress measurement using a microcantilever” by M. Godin et al, in Applied Physics Letters, 79(4), pp 551-553, 23 Jul. 2001. Methods that also include four-point bending techniques are already known from the publication entitled “Adhesion and de-bonding of multi-layer thin film structure” by Dauskardt et al in Eng. Fract. Mech, 61, pp 141-162, 1998.However, the above cited tests are destructive: they require the use of samples of a certain size and said samples are unsuitable for use after testing. Moreover, these known techniques are not well-suited to multi-material stacks involving thin films of the kind encountered in microelectronic devices.
Other adhesion-strength measurement techniques are known in which the interface between two materials is supplied with power from a laser.
For example, U.S. Pat. No. 4,972,720 describes a technique for assessing adhesion strength at an interface by heating the interface and evaluating the temperature at which thermal debonding occurs. The interface can be heated by applying laser energy over a relatively extended period (e.g. 5 seconds), and debonding can be detected by various techniques including using an acoustic sensor. The above cited patent U.S. Pat. No. 4,972,720 teaches that interfaces having lower adhesion strength produce noisier debonding events.
There are various disadvantages of the method described in U.S. Pat. No. 4,972,720. Firstly, the materials under test may experience an undesirable change in properties when they are heated. For example, a heated steel sample could undergo a martensitic transformation. Secondly, this known technique will only work when applied to the interface between two materials having different coefficients of thermal expansion.
U.S. Pat. No. 5,838,446 describes a technique for determining the strength of adhesion of a transparent coating provided on an opaque basecoat. Laser energy is used to ablate the basecoat at the interface between the basecoat and clearcoat such that a blister forms. Adhesion strength is determined from various parameters including the size of the blister, and a critical energy value at which a crack begins to propagate from the blister. A single IR (λ=1053 nm) laser pulse of 50 ps width is used to irradiate a single spot on the sample, then the energy value of the laser is changed and a new spot irradiated. At each spot, the radius of the blister that is formed is measured. The radius values from a series of spots are plotted on a graph in order to estimate a radius value for the critical energy. Calculation of the adhesion strength uses this estimated radius value.
There are disadvantages, too, with the method described in U.S. Pat. No. 5,838,446. Notably, ablation of materials is likely to generate dust, which will be undesirable in many manufacturing environments, e.g. in the microelectronics industry. Moreover, this technique requires use of a particular sample geometry which involves handling the sample to an extent which may be undesirable (e.g. if the sample is a semiconductor wafer being used to manufacture microelectronics devices). Further, in order to calculate a single adhesion strength value, measurements must be taken at a number of locations on the sample and the resultant data combined—leading to undue complication and a relatively lengthy calculation time.
U.S. Pat. No. 5,438,402 describes a laser spallation technique for determining tensile strength at the interface between a substrate and a coating. In such laser spallation techniques, a mechanical impulse is applied to the substrate and coating. In order for the mechanical impulse to be transmitted to the substrate/coating, it is necessary to provide an energy-absorbing coating on the free surface of the substrate (the surface remote from the coating), and a confinement plate on the energy-absorbing layer. A pulse from a laser is used as the energy source in this arrangement. Adhesion strength is calculated based on the movement of the free surface of the coating.
It is an object of the present invention to provide a new technique for evaluating adhesion strength at an interface between two layers, for example between layers of two different materials in a multi-material stack.
Preferred embodiments of the present invention provide an adhesion-strength evaluation method which is simpler to implement than the methods known heretofore. More particularly, in these preferred embodiments, a value for adhesion strength can be calculated without the need to process data relating to a set of locations on the sample or the need to use a complicated confinement structure.
Preferred embodiments of the invention allow adhesion strength to be measured at discrete locations over a surface.
The present invention provides a method of measuring adhesion strength at the interface between two layers, in which method a laser pulse is caused to impact directly on one of said two layers so as to produce a shock wave at the interface, and a sensor detects rupture of the interface (debonding). The adhesion strength at the interface between the two layers is determined based on the energy and wavelength of the laser pulse required to produce the rupture of the interface.
This technique is extremely simple to put into operation. Adhesion strength can be calculated based on parameters relating to a laser impact at a single point, thus simplifying and speeding up calculation. Moreover, this technique merely involves placing of a wafer, or other multi-material sample, on a pedestal; there is no need to arrange the sample relative to energy absorption layers or confinement plates.
The present invention allows adhesion strength to be measured in a manner which is not globally destructive of the tested sample and without generation of dust. Although there is disruption of the interface at the point(s) where testing is performed, the remainder of the sample is still useable. Thus, the present method is well-suited to testing adhesion strength between layers on a semiconductor wafer which is to be cut up into discrete devices.
Various types of sensor can be used to detect the rupture of the interface between the two layers undergoing test. In preferred embodiments of the invention the sensor is an acoustic sensor or an x-ray reflection device.
The invention and additional features, which may be optionally used to implement the invention to advantage, are apparent from and elucidated with reference to the drawing described hereinafter.